Molecular dynamics studies on monolayer of cetyltrimethylammonium bromide surfactant formed at the air/water interface

An atomistic molecular dynamics (MD) simulation has been carried out to investigate the structural and dynamic properties of a monolayer of cationic surfactant cetyltrimethylammonium bromide (C16TAB) formed at the air/water interface. The simulation was performed at room temperature, and two systems I and II containing different surface areas 45 and 68 Å2 per molecule, respectively, were selected. Through the radial distribution function (RDF) among the headgroups, water molecules and bromide ions, the thickness of the adsorbed layer was estimated. The hydrocarbon chain structure was analyzed by taking into account the length distributions between headgroups and different methyl(ene) groups in the chain, and it is shown that two systems have a big difference in the structures of the monolayer, especially for the last several methyl(ene) groups normal to the interface. The disordered monolayer for system I is proved by the orientation correlation function and the fractions of gauche conformations in the chain. It indicates that the structure of monolayers depends on the vertical position with respect to the interface. We found that water molecules around the headgroups at the interface have smaller diffusion coefficient than those in the hydrocarbon tails or in the bulky water. It is shown that the diffusion of water molecules mostly depends on the surroundings, and the electrostatic and H-bonding interactions are the reason that water molecule has small diffusion coefficient in the chains of the monolayer.